At the present time anesthetics, drugs which induce loss of awareness, are often used for surgical operations. A general anesthetic should cause a progressive depression of the central nervous system and cause the patient to lose consciousness. In contrast, a local anesthetic will affect sensation at the region to which it is applied.
Generally the patient, prior to the surgical operation, is anesthetized by a specialized medical practitioner ("anesthesiologist") who administers one or more volatile liquids or gases, such as nitrous oxide, isoflurane, desflurane, ethylene, cyclopropane, ether, chloroform, halothane, etc. A preferred gas is Ciboflorane (TM Abbott) which, however, may sometimes not be used because of its cost. Alternatively, non-volatile drugs may be administered by injection or intravenous infusion, such drugs including pentothal, propofol, evipal and procaine.
The objectives of a correctly administered general anesthetic include:
(1) the patient's movements are blocked; PA1 (2) the patient should feel no pain; PA1 (3) the patient should be unaware of and not remember the operation; PA1 (4) the anesthesia should not lower blood pressure to a dangerous extent (generally below 50 mm Hg for mean arterial pressure (MAP)).
A skilled anesthesiologist may monitor the vital signals (breathing, blood pressure, etc.) of the patient to determine if more, or less, anesthetic is required. Often he/she looks into the patient's eyes to determine the extent of the dilation of the pupils as an indication of the level (depth) of the effect of the anesthesia (called "plane of anesthesia"). However, there may be a number of problems with such complete reliance on the availability, skill and attention of the anesthesiologist. In some operations, such as some heart surgery, the head is covered so that the patient's eyes cannot be viewed. Some operations may be prolonged, for example, 10 to 15 hours, so the attention of the anesthesia nurse or anesthesiologist may flag or fail.
An anesthesiologist may not be available in some situations and localities. For example, in an emergency or battlefield situation, a general physician or surgeon may be present and able to perform an operation, but an anesthesiologist is not available. Similarly, in isolated geographic locations, especially in second- and third-world countries, it may be impractical to move the patient to a hospital center for an operation. A physician or surgeon would be able to perform the operation if there were some way to safely anesthetize the patient.
It has been suggested in prior patents that some of these problems would be avoided by having a computer system determine the best amount of anesthetic. However, such computer determinations have not been accepted and are not being used.
U.S. Pat. No. 2,690,178 to Bickford relates to an automatic system to apply anesthetics to a patient by monitoring the patient's brain waves. Bickford used the integrated potential output of the cortex. U.S. Pat. No. 4,280,494 to Cosgrove et al is entitled "System For Automatic Feedback-Controlled Administration of Drugs". It discloses a closed loop control of anesthetics, such as liquid thiopental, and the use of EEG electrodes. The EEG measure used is "EEG power response" ("total power output"). However, based upon present knowledge, it is believed that the use of the single measure of integrated cortex output or total power, as in Bickford and Cosgrove, does not a provide a reliable control signal for applying a general anesthetic. Many anesthetics actually cause an increase in power of the cortical EEG and, in some instances, the nature of power changes depending upon electrode position. Further, it must be emphasized that not only do different anesthetics have different effects upon the EEG, but those effects may vary from patient to patient as a consequence of different premedications and/or different biochemical sensitivities.
The inventor's prior U.S. Pat. No. 4,557,270, entitled "Electroencephalographic System For Intra-Operative Open-Heart Surgery", incorporated by reference herein, describes an electroencephalograph (EEG) system used intra-operatively in cardiovascular (open-heart) operations using a heart-lung machine (cardiopulmonary by-pass) such as heart valve replacement surgery. That system, called "CIMON" (Cardiovascular Intraoperative Monitor) is presently being sold by Cadwell Laboratories, Kennewick, Washington, and has been successfully used in many heart operations. However, the CIMON system, with its attention to cerebral ischemia related to the output volume of the heart pump, etc., is not used in general surgery and is not used to control the application of an anesthesia.
In Chamoun U.S. Pat. No. 5,010,891 EEG potentials from a group of healthy surgical patients are recorded (col. 14, lines 32-34). A "reference array" is obtained of the most significant locations and an "autobispectral density index" is defined based on the recordings from a normal group. Each normal group index is then compared to the index of the patient under review. However, the comparison of individual patients with a normal group, in itself, is not believed to provide reliable information in the surgical context of determining if a patient will be sufficiently anesthetized.
In Silberstein U.S. Pat. No. 4,869,264 light flash stimuli are given to a patient before and after the patient is subjected to a general anesthetic. The light flashes are at two or three frequencies and the patient's brain waves are detected by EEG. Silberstein mentions automatic control of an anesthetic but, for safety reasons, says that his system should be used to monitor patients.